1. Technical Field
The present invention relates to a data hiding method for hiding message data into media data and a data extracting method for extracting hidden data.
2. Prior Art
With the development of multimedia society, large quantities of digital video and audio information have been circulated on internet systems or as CD-ROM software. For digital video and audio information, anybody can easily create a perfect copy without degradation, so the illegal use and the protection of the copyright are becoming problematic. In order to prevent a third party from illegally copying media data such as video and audio data, the technique of hiding additional information, such as the signature of a creator (author), into original media data is becoming the focus of attention. When digital video data or other similar data is illegally copied, it can be known whether or not the copy is an illegal one by confirming the signature hidden in the copy and specifying the source. A hiding technique such as this is called data hiding. FIG. 1 shows a half-tone image obtained when digital data is displayed on a display. In the media data of FIG. 1(a) which is a digital image, messages, such as a nurse, a river, kindergarten pupil, and birds, have been hidden as shown in FIG. 1(b). Media data is obtained by segmenting an image (obtained, for example, from a photograph) into very fine parts and numerically expressing brightness and a hue for each part. At that time, the original numerical value of the image is slightly changed intentionally. If there is a small change in the numerical value, there will be almost no disturbance in the image and humans will not sense the disturbance. If this nature is skillfully utilized, entirely different information (message data) can be hidden in original video. This message data may be any information, for example, lattice patterns, rulers, or signatures of video creators. The message data hidden in media data can be extracted by processing it with a special program. Therefore, based on the extracted message data, it can be checked whether the media data has been altered.
Incidentally, the Moving Picture Experts Group (MPEG) is well known as one of the methods of compression for motion images (video data). In the case where some additional information is put into an MPEG video bitstream, a method of hiding additional information into a user data field has generally been employed. In such a method, however, the field can be easily separated from the media data, so there is the problem that the detection and removal of additional hidden information are easy.